To examine whether physical activity on prescription in routine primary care patients would influence physical activity level and quality of life 6 months later. In 2001-2003, 13 Swedish primary health care units took part in an uncontrolled clinical study. If a patient in primary health care needed physical activity preventively or for treatment of a disease and patient-centered motivational counseling found physical activity to be suitable, individualized physical activity could be prescribed. Patients (n=481) of both sexes and all ages [75% women, mean age 50 (12-81)] participated in the study. Self-reported physical activity, readiness to change to a more physically active lifestyle and quality-of-life data were collected through questionnaires. The follow-up rate was 62% at 6 months. Intention-to-treat analysis showed a significant increase (P<0.01) in self-reported physical activity level, the stages of action and maintenance of physical activity as well as quality of life. Physical activity level, stages of change and quality of life increased analogically, indicating that physical activity on prescription may be suitable as a conventional treatment in an ordinary primary health care setting to promote a more physically active lifestyle.